The present invention relates to sensing systems which remotely indicate a sensed parameter, and in particular to such systems used to sense the level of liquid in a container.
In the past, the level of a liquid in a holding tank in recreational vehicles has been measured and monitored by various means. One particularly popular approach has been to install electrically conductive probes through the side walls of the container at appropriate heights, and to measure the electrical conductivity of the probes with respect to a common probe in order to determine which of the probes is below the liquid level in the container. Typically, five probes have been used per container, a common probe and a sensing probe for the 1/4, 1/2, 3/4, and Full levels. In the past, conductors from each of these probes have been routed to a monitor panel mounted in a convenient location inside the recreational vehicle. Typical wire length for each of these five conductors is often in the neighborhood of 25 feet.
Liquid level monitoring systems of the type described above have a number of significant drawbacks. First, the conductivity of liquids typically held in recreational vehicles (such as waste, sewage, and fresh water, for example) varies widely, by more than ten to one. This variation in liquid conductivity makes it difficult to select a single value of conductivity which can be used to distinguish clearly between a sensing probe above the liquid level and a sensing probe below the liquid level. Furthermore, when liquids are splashed against the side walls of the container, false or misleading indications can result. For example, the full indicator may be illuminated while the 3/4 full indicator is not.
A second important drawback is that the system described above requires a relatively large number of conductors to connect the probes to the electronics of the mounting panel. In the past, five separate wires have been used, which require a considerable amount of labor to interconnect and which bring with them potential for error, potential for failure, and difficulty in routing and concealing the wires. In addition, the use of five separate wires increases the cost of the system.